Added support for PG's default_statistics_target

Yorick Peterserequested to merge
pg-default-statistics-target into master

This allows us to set/manage this setting via Chef, instead of having to hack it into the runtime.conf file directly on the primary.

Adding this setting is one of the TODOs listed in https://gitlab.com/gitlab-com/infrastructure/issues/4429#note_82560203 for today's incident issue.

cc @marin

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